Acrotrichis sitkaensis

My first featherwing beetles were also the first beetles I found with my new suction sampler. At about ¾mm long it was pretty tough seeing all the detail referred to in the key (I used Duff) but I initially thought atomaria fitted both the male and the female. They were suctioned from my lawn, together, when briefly trying out my new kit.

Thanks to Steph Skipp who looked at the post on BlueSky and pointed out that while the male looks good for atomaria, the tubes on the female are a bit thick so more like sitkaensis. I must admit I had assumed that as I found them together and they were my first Ptiliids, they'd be the same species, though I should have known better! Unfortunatley I didn't take the usual care to keep both specimens separate prior to dissecting them so am not entirely clear which photos relate to which beetle (although I did note that the one which in my last photo had its wings protruding beyond the elytra was the female). I also kicked myself for not taking accurate measurements of both beetles - they appeared to be the same size but as atomaria normally smaller than sitkaensis a more accurate measurement of both might have helped clear things up. Unfortunately getting at their abdomens wrecked the beetles so no chance of getting measurements now.

female Acrotrichis sitkaensis showing wing and spermatheca (2 views), North Elmham (Norfolk, UK), 1st March 2025

I clearly needed to get out and find some more and a few days later this is exactly what I did, finding three externally-similar Acrotrichis together in my lawn. Again I only suctioned for a very short period so they must be abundant here! This time I took care to keep the specimens separate from one another so I knew which photos related to which individual. All three turned out to be males but once again two species were involved. One matched atomaria but the other two were sitkaensis (I think this is the correct ID but I acknowledge that the spines on the sixth visible sternite appear to be arranged more unevenly than the two tidy rows shown in Duff).

male Acrotrichis sitkaensis showing sixth visible sternite (3 views) and aedeagus, North Elmham (Norfolk, UK), 8th March 2025

second male Acrotrichis sitkaensis showing wing and aedeagus, North Elmham (Norfolk, UK), 8th March 2025

Again I forgot to take measurements of the whole insects until I'd dissected them so it was too late, but I did carefully compare the size of the aedeagus and the atomaria was distinctly smaller than the two sitkaensis as shown below.

male Acrotrichis atomaria (left) and male Acrotrichis sitkaensis (right) aedeagi with scale bar (same on each), North Elmham (Norfolk, UK), 8th March 2025

Suction sampling must be the way to find these as I soon found another one this way at a different location. This one was 0.80mm long. The spermatheca was very fragile and fell apart when I attempted to clean it.

female Acrotrichis sitkaensis showing spermatheca (2 views, broken in second), Wendling Beck Environment Project (Norfolk, UK), 12th March 2025